UWB wireless broadcasts are capable of carrying huge amounts of data up to 250 feet with extremely little transmit power and high immunity to interference and multipath fading. Indeed, the spread spectrum characteristics of UWB wireless systems, and the ability of the UWB wireless receivers to highly resolve the signal in multi-path fading channels make them a desirable wireless system of choice in a wide variety of high-rate, short- to medium-range communications. The ability of UWB systems to locate objects to within one inch attracts the military, law-enforcement, and rescue agencies. Other applications include broadband sensing using active sensor networks and collision-avoidance. The circuit techniques that are used to realize different circuit components in a UWB transceiver are quite different from those used in current narrow bandwidth RF technology. This notion provides an incentive to investigate the design of novel circuit topologies that achieve a gain-for-delay-tradeoff without affecting bandwidth, thus operating at substantially higher frequencies than conventional circuits.
The main challenge to design wideband, e.g., ultra-wideband (UWB), transceivers is to satisfy gain, NF, reverse isolation, and linearity requirements over a wide bandwidth (e.g., 7.5 GHz in a UWB wireless system). Recently, different circuit techniques have been proposed to achieve wideband operation of the RF front-end. [KIM05] presented a resistive feedback amplifier covering the UWB lower frequency band, i.e., 3-5 GHz. The input matching and gain of the circuit in [KIM05], however, drops at higher frequencies due to the dominating effects of the device's parasitic capacitances. A resistive feedback amplifier in bipolar technologies proposed in [LEE05] covers the entire UWB frequency band with minimum NF of around 3.2 dB. Another solution to the UWB LNA design is to transform conventional narrowband techniques to wideband by using higher-order bandpass filters to achieve required wideband input matching [BEV04], [ISM05]. However, the overall gain response and NF of the wideband LNA proposed in [BEV04] vary rather significantly across the UWB frequency band due to existing mismatch from a frequency-dependent inductive-degenerated 50Ω seen from the input of the transistor. The circuit in [ISM05] is designed with bipolar devices. The use of a distributed topology provides the required wideband operation, as shown in [ZHA05], [HEY05]. The main advantage of distributed architecture is its intrinsic wideband matching characteristics [HEY05]. However, distributed architecture circuits are prone to high power dissipation and take up larger chip areas.